Semiconductor nanocrystals (NCs), often referred to as nanocrystal quantum dots (NQDs), are of interest for their size-tunable optical and electronic properties. Intermediate between the discrete nature of molecular clusters and the collective behavior of the bulk, NQDs are unique building blocks for the bottom-up assembly of complex functional structures. NQDs can be conveniently synthesized using colloidal chemical routes such as solution-based organometallic synthesis approaches described by C. Murray et al., J. Am. Chem. Soc., 115, 8706 (1993) or by Peng et al., J. Am. Chem. Soc., 123, 183 (2001), such references incorporated herein by reference. Generally, these procedures involve an organometallic approach. Typically these chemical routes yield highly crystalline, monodisperse samples of NQDs. Because of their small dimensions (sub-10 nm) and chemical flexibility, colloidal NQDs can be viewed as tunable “artificial” atoms and as such can be manipulated into larger assemblies engineered for specific applications.
A significant challenge for obtaining stable optical properties and realizing optical applications of quantum dots is to incorporate the NQDs into a suitable transparent host matrix. Early attempts involved the direct growth of the nanoparticles within glassy matrices; however, the resulting materials were plagued by poorly controlled surface passivation, low filling factors, and large size dispersities.
More recently, researchers have sought to decouple the synthesis of the nanoparticles from the fabrication of the composites. Selvan et al., Adv. Mater. v. 13, pp. 985-988 (2001) describe octylamine-passivated semiconductor quantum dots transferred into butanol prior to sol-gel processing with resultant volume fractions or loadings of only up to about 0.1 percent. Sundar et al., Adv. Mater., v. 14, pp. 739-742 (2002), describe incorporation of NCs wherein the surface-passivating ligands are replaced with tris(hydroxylpropyl) phosphine to stabilize the NCs in polar solvents such as ethanol and to provide hydroxyl groups which can be reacted into a titania sol-gel matrix. Volume fractions or loadings of as high as 10 to 12 percent were reported.
Despite the gradual progress, problems have remained. For example, the ligand exchange process used during fabrication of colloidal nanocrystal/sol-gel composites inevitably leads to a reduction in the photoluminescence quantum yields (PL QYs) of the colloidal nanocrystals. In addition, capping groups have varying affinities for different NQD compositions and shapes, requiring a careful selection of ligand each time these parameters are changed. After long and careful research, a different approach has now been developed for the preparation of colloidal nanocrystal-containing composites.
It is an object of the present invention to provide a new process for preparing solid composites including colloidal nanocrystals and to provide the solid composites from such a process.
It is another object of the present invention to form solid composites with high volume loadings of the colloidal nanocrystals.
Still another object of the present invention is solid composites including colloidal nanocrystals where the solid composites are characterized by high refractive indices.
Still another object of the present invention is the preparation of alcohol-soluble colloidal nanocrystals.